This invention relates to processing of semiconductor wafers and, more particularly, to a process for accurately controlled, high temperature drive-in diffusion of dopant impurities into a semiconductor wafer.
Diffusion is a well-known technique for producing impurity doped regions in semiconductor wafers. In the conventional approach, a batch of wafers is placed in a diffusion furnace and exposed to a gaseous form of the impurity species which is deposited on or near the wafer surface. The wafer is then subjected to drive-in diffusion, usually in another diffusion furnace. An inert gas at a temperature of 900.degree. C. to 1050.degree. C. flows through the furnace and causes the impurity atoms to diffuse into selected regions of the wafer not protected by oxide layers. The depth of diffusion depends on impurity concentration, temperature and time in the furnace. For the above temperature range, diffusion time is typically on the order of thirty minutes.
Long-standing problems with diffusion furnaces have been poor impurity uniformity over the surface of the wafer and poor control of impurity dosage. As integrated circuit devices become smaller and more dense, these problems limit the usefulness of the diffusion furnace.
Implantation of ions into semiconductor wafers is a doping technique which alleviates these problems. Doping uniformity and dosage can both be accurately measured and controlled. However, ion implantation is limited, due to energy considerations, in the depth to which impurity dopants can be deposited. To address this problem, a technique involving predeposition ion implantation followed by drive-in diffusion has been developed. Ions are implanted at moderate energies into the near surface region of the wafer and then processed in a diffusion furnace in order to drive the impurities to a greater depth and to anneal the radiation damage due to the implant. This technique permits good control of dosage and uniformity.
One drawback of drive-in diffusion, whether employed after vapor deposition or ion implantation, is the time required for the process. The diffusion process typically requires thirty or more minutes, and additional time is required for transferring the wafers in and out of the furnace. In a commercial semiconductor processing environment, any reduction in processing time is highly advantageous. It is known that the rate of diffusion of impurities into semiconductor material increases with temperature and, more particularly, that the diffusion constant is an exponential function of temperature. However, due to the nature of diffusion furnaces, it has not been possible to increase the operating temperature in order to reduce the processing time. Diffusion furnaces have large thermal masses and are maintained at operating temperature. The movement of wafers in and out of the furnace is done slowly to avoid wafer cracking and breakage due to thermal stresses. This creates an uncertainty in the time that the wafers are exposed to the high temperature. Differences in time at high temperature produce differences, or errors, in the diffusion of impurities. An increase in the operating temperature of the diffusion furnace increases these errors to an intolerable level since the processing time is decreased but the source of timing error is unchanged.
It is a general object of the present invention to provide a process for high temperature drive-in diffusion.
It is another object of the present invention to provide a process wherein the rate of drive-in diffusion is increased.
It is yet another object of the present invention to provide an accurately controlled process for high temperature drive-in diffusion.
It is still another object of the present invention to provide a process for drive-in diffusion wherein a semiconductor wafer is exposed to a planar radiation source.